generate sequential Number when a sheet is inserted

Hi,
I have a file in which i prepare Delivery Challan n invoice. Many times order has to be sent in parts and each time a part is sent, Delivery Challan Number changes while Invoice Number remains same.

For each order, there is a single Reg No. and a single invoice number but Delivery Challan number may vary from 1 to more.
my delivery challan Number format is: "Reg No." followed by "/" and then the number "001" , "002" etc.

For eg. I have an order of 9 products (I register order in my company as Reg. No).
In 1st batch 3 products are sent (1st Delivery challan and invoice is generated:- D.C No.: Reg No/001).
In 2nd batch other 4 products are sent (2nd Delivery challan and invoice is generated:- D.C No.: Reg No/002) and
remaining in 3rd batch (3rd Delivery challan and invoice is generated:- D.C No.: Reg No/003).

I would prefer if Delivery Challan number is generated in sequence automatically for each such sheet (sheet names vary because I rename the sheet with the date of generating invoice)

I have attached the sample file for better understanding.

Thanx a lot..